Iran's government on Thursday made clear it has no interest in direct talks until the U.S. eases sanctions that have been squeezing Iran's economy. But the Obama administration isn't budging and says the ball is in the Iranians' court.

The suspicion that Iran wants to make a nuclear weapon is the rationale for the sanctions as well as for veiled threats of U.S. or Israeli military action if those sanctions fail.

Iran's perceived nuclear aspirations are also the subject of a global effort that keeps popping up in the news: the game of "nuclear keep-away" to keep Iran from buying or manufacturing centrifuges, the machines that make uranium suitable for a bomb.

"We call it the long pole in the tent," physicist David Albright, a former United Nations weapons inspector, tells All Things Considered host Robert Siegel. "Getting the wherewithal to make the weapon-grade uranium or the separated plutonium is harder than learning how and assembling everything you need to know to make the nuclear weapon itself."

Separate Your Isotopes

Most uranium is useless for nuclear fuel or weapons. Less than 1 percent of it is the light, radioactive isotope uranium-235 that's used for power plants and bombs.

But if you gasify uranium with fluorine and put it in a rapidly spinning centrifuge, you can separate the nuclear wheat from the chaff. As the centrifuge spins, the heavier, nonradioactive U-238 moves to the centrifuge wall, while the U-235 moves toward the center, explains Houston Wood, a University of Virginia engineering professor who worked on gas centrifuges at the U.S. Department of Energy.

Ultimately, two streams are removed: one of enriched U-235, the second depleted in U-235. And when you take the material that has more 235 and conduct the process over and over again, you eventually get uranium that is 90 percent U-235: weapons-grade uranium.

Build Lots Of Centrifuges — Fast Ones

Getting that weapons-grade product takes thousands of centrifuges, spinning as quickly as possible and arranged in what's called a cascade.

How fast they can go depends on what they're made of. An aluminum centrifuge can only spin at about 350 meters per second before it bursts, explains Wood. Another material, maraging steel, can spin at 450 meters per second.

Even more appealing is carbon fiber, used to make products like golf club shafts and bicycle frames. Centrifuges made of carbon fiber can reach speeds of 1,000 meters per second.

That's where the game of nuclear "keep away" comes in. Much of the global effort is aimed at blocking Iran from getting the highly specialized materials needed to make those fast-spinning centrifuges.

Gather The Right Materials

"Iran successfully bought 70 tons of maraging steel, enough in theory for over 30,000 centrifuges, from Britain in the 1990s," says Albright. "The Brits caught them eventually and kept them from buying more."

The Department of Justice says the Iranians are still shopping for maraging steel. One of the department's hottest cases involves Iranian citizen Parviz Khaki, who the U.S. says tried to buy 20 tons of the material, allegedly with the help of a Chinese citizen.

While the steel has many "innocent" uses, says DOJ prosecutor Steven Pelak, the attempted transaction just didn't add up. In particular, the Chinese company trying to buy the material was a toy company — and "toy companies don't need 20 tons of maraging steel," Pelak says.

Khaki and the Chinese citizen were indicted in July and are now in custody in the Philippines, fighting extradition to the U.S.

Carbon fiber centrifuges spin even faster than those of maraging steel, but making the fiber is a complex process, says Zsolt Rumy, who runs a St. Louis company that makes carbon fiber. It also involves navigating a specialized industry where people know people.

In December, the Justice Department arrested an Iranian-American named Reza Hamid Hashemi in New York. The U.S. says Hashemi had been shopping for a type of carbon fiber used mostly by aerospace companies but also for centrifuges.

The Global Game Of 'Keep-Away'

U.S. sanctions against Iran have made the global game of nuclear keep-away easier. The measures are so broad, even shopping for a carbon fiber winding machine to make golf club shafts could run afoul of them.

But how successful can the U.S. really be at keeping these specialized products out of hands it deems dangerous?

In the case of Iran, Albright says the odds are 50-50. "What you see is Iran's procurements are often detected and stopped. ... In the end, though, what you're trying to do is buy time. You're trying to make it harder for them to succeed," he says.

"You want to disrupt or create bottlenecks in their ability to develop centrifuges and deploy them. You want to limit the number they can build," he says. But ultimately, he says, "you can delay their program, but you can't stop it."

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Transcript

ROBERT SIEGEL, HOST:

This is ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. You are holding a gun against Iran, saying talk or you'll fire - so reads a statement issued today by Iran's Supreme Leader Ayatollah Ali Khamenei. It comes in response to a speech earlier this week from Vice President Joe Biden suggesting the U.S. would be open to direct talks with Iran over its nuclear ambitions.

SIEGEL: The Supreme Leader's answer, in short, talks will not solve any problems. Iran's President Mahmoud Ahmadinejad echoed that sentiment later in the day while visiting Cairo. He made clear that his government has no interest in direct talks until the U.S. eases sanctions that have been suffocating Iran's economy.

CORNISH: But the Obama administration isn't budging. This afternoon, State Department spokeswoman Victoria Nuland said the ball is in the Iranian's own court.

VICTORIA NULAND: The burden of these sanctions could be eased if they made a decision to engage with us substantively. We've always said that action on the Iranian side would be matched by action on our side. So it's really up to Iran to engage if it wants to see sanctions eased.

SIEGEL: The suspicion that Iran wants to make a nuclear weapon is, of course, the rationale for those tough sanctions, as well as for veiled threats of U.S. or Israeli military action if those sanctions fail to persuade. It's also the subject of a global effort that keeps popping up in the news every few weeks, a game of nuclear keepaway, an effort to keep Iran from buying or making the machines that make uranium suitable for a bomb - centrifuges.

DAVID ALBRIGHT: We call it the long pole in the tent. Getting the wherewithal to make the weapon-grade uranium is harder than learning how and assembling everything you need to make the nuclear weapon itself.

SIEGEL: That's David Albright. He's a physicist and a former U.N. inspector. Most uranium is useless for nuclear fuel or weapons. Less than 1 percent is the light radioactive isotope U235. But gasify uranium with fluorine, put it in a centrifuge, and as Professor Houston Wood of the University of Virginia explains, you can separate the nuclear wheat from the chaff.

HOUSTON WOOD: The centrifuge is spinning very rapidly and then the gas will spin at the same speed as the wall is spinning, and the heavier isotope, uranium 238 will tend to move towards the wall and the lighter isotope will tend to move towards the axis. And then you remove uranium that has been enriched in uranium 235 and the other stream is depleted in uranium 235.

SIEGEL: Houston Wood, who worked on gas centrifuges at the U.S. Department of Energy, is talking about a very slight degree in enrichment when you do that. But if you take the stuff that has more U235 and do this over and over and over again, eventually, you can get uranium that is 90 percent U235, weapons-grade uranium. That takes thousands of centrifuges arranged in what's called a cascade.

David Albright says the Iranians started shopping for the parts to do all this as early as 1985.

ALBRIGHT: It's when they first showed up in Europe looking to buy equipment and their philosophy at the time was let's go buy what we need. They went out and bought all kinds of equipment in Europe.

SIEGEL: But they couldn't make it work. In the early 1990s, Albright says, they turned to A.Q. Khan, the European-trained physicist who had made Pakistan's bomb.

ALBRIGHT: And they decided to just go out and buy 500 of what were called these P1 centrifuges from the Khan network.

SIEGEL: P is for Pakistan. Over the years, Iran has told the International Atomic Energy Agency that with the centrifuges it has, it has enriched uranium to the 5 percent level, that's low enriched uranium, to make fuel for a nuclear power plant. They also reported enriching some uranium to 20 percent, which they say is for a research reactor.

And according to a recent AP story, they evidently intend to enrich more uranium to that level. They have never said they intend to go for 90 percent. But it shouldn't be that hard. It takes far more effort to get to 20 percent enrichment from less than 1 percent than it would take to go from 20 percent to 90 percent. The way you enrich more uranium is to have more centrifuges and also to make them go faster.

Professor Houston Woods says how fast depends on what they're made of.

WOOD: If you make them out of aluminum, they can only go to about 350 meters per second and that's because if you spin it faster, the metal will just burst. And there's another material called maraging steel and that can be used to make centrifuges and it can go up to about 450 meters per second. And after that, there are composite materials like carbon fiber that are used to make centrifuges. And the limit on that is 1,000 meters per second.

SIEGEL: So part of the game of keep-away has been trying to block Iran from getting these highly specialized materials with which they could make centrifuges spin faster. Houston Wood mentioned maraging steel, a steel that is hardened with nickel and other metals. David Albright says the Iranians went shopping for it in Britain.

ALBRIGHT: Iran successfully bought 70 tons of maraging steel, enough in theory for over 30,000 centrifuges, from Britain in the 1990s. The Brits caught them eventually and kept them from buying more.

SIEGEL: According to the U.S. Department of Justice, the Iranians are still shopping for maraging steel. Prosecutor Steve Pelak says one of the hottest cases the department has brought involves an Iranian named Parviz Khaki. He tried to buy 20 tons of the maraging steel, allegedly with the help of a Chinese citizen. Last July, the two were indicted.

STEVEN PELAK: The goods were maraging steel and other devices that can be used in many, many common uses in the scientific communities. But we knew from the initial calls to the company that something just didn't make sense.

SIEGEL: In particular, the Chinese company that was trying to buy the strengthened steel in the U.S. was a toy company.

SIEGEL: That would make one hell of an Erector set. The case against Parvis Khaki, who was arrested in the Philippines and remains in custody there fighting extradition to the U.S., reveals an interesting fact about the game of nuclear keep away. Justice Department officials say Khaki found that Chinese maraging steel was shoddy. The good stuff is made here, but shopping for it in the U.S., you find yourself in a pretty small market.

PELAK: The companies that make and sell these goods legitimately, they know who their customers are. They know who's buying things and why. So when they're approached by somebody that's out of left field that just doesn't make sense.

SIEGEL: In this case, the company tipped off the government and prosecutors set up a sting operation. Centrifuges made of carbon fiber spin even faster than maraging steel, but David Albright say they also pose problems.

ALBRIGHT: When you use carbon fiber in the rotor, I mean, it involves winding this very strong fiber around, it's called a mandrill, just a cylinder. And it's hard to figure out what pattern to use.

SIEGEL: And before you even wind the carbon fiber, somebody has to make it.

ZSOLT RUMY: Making carbon fiber is just kind of like sex. You can read about it and talk about it all you can, but you don't know what it's like until you actually do it.

SIEGEL: That's Zsolt Rumy, who runs a company called Zoltek in St. Louis that makes carbon fiber. Zoltek has nothing to do with this case, but Rumy told me about this product that can be used to make such fast-spinning centrifuges and also it can be used to make golf club shafts and bicycle frames. Once again, it's a specialized industry and people know people.

RUMY: We know our customers because applying carbon fibers and using it in various processes is difficult.

SIEGEL: In December, the Justice Department busted an Iranian-American named Reza Hamid Hashemi when he stepped off a plane in New York. The U.S. says Hashemi had been working through a Turkish partner, shopping for carbon fiber in the U.S. He was interested in a type of fiber called IM7, that's used mostly by aerospace companies, but it's also advertised for centrifuges for nuclear enrichment.

Hashemi has refused to comment through his lawyer. He allegedly came to the U.S. to see a carbon fiber winding machine and that turned out to be another sting. The global game of nuclear keep-away has gotten a lot easier with the new U.N. sanctions against Iran, but how successful can the U.S. really be at keeping these specialized products out of hands it deems dangerous?

I asked David Albright: Who's favored to win in this game of keep-away; the would-be nuclear proliferator or the anti-proliferation effort?

With export controls, the proliferator. In the case of Iran, with the U.N. Security Council resolutions that made it easier to police this, I think it's 50/50. And what you see is Iran's procurements are often detected and stopped; there interdictions of nuclear related items. What you're trying to do is buy time. You want to limit the numbers that they can build so you can delay their program. But you can't stop it.

Which is one reason we're hearing a lot about the alternatives to nuclear keep-away: diplomacy, sanctions and the threat of force. And we're likely to hear a lot more about all of the above in the coming months. Transcript provided by NPR, Copyright NPR.